Drew-
The only way to get "turn by turn" mapping of roads is by buying it.
Usually from each state highway/transportation department. For various legal
reasons roads are officially "mapped" or "demapped" and the first place to
get official record of that is the appropriate state agency in each state.
AFAIK there is no federal central version of this data. While the USGS maps
are updated "whenever" and often with aereal updates, the state planimetric
data are still sold--not freely given for commercial users--which comes back
to DeLorme's old problem, they have to BUY the data from all 50 states in
order to get it annually updated. Same problem for any user, really.
Of course that still leaves the problem of incorrect state
mapping/demapping. Sometimes roads are mapped, but never built. Or demapped,
but still in existence. For decades and decades, for all sorts of odd
political reasons. That's something the mapping companies only will get from
ground conformation via user feedback, and most of the "new" companies no
longer care about that. Cartographers traditionally *gave* copies of their
new maps to users who reported field errors. Don't expect that from
"computer mapping" companies, save your stamps. DeLorme even includes
errors--intentionally--to prove data was taken from their products.